File-based configuration

[zanieb]

We should support configuration via a file, but what this looks like exactly is not yet determined.

[DanCardin]

Please dont use dirs, as they've explicitly rejected the idea of supporting XDG on macos, which is really annoying to the people that care, and irrelevant to the people that dont care. You'll eventually (or immediately) get requests to do so, and will have to work around their weird stance. Nushell has had a many years long open issue as a result of their early choice to use app_dirs.

I've had good success with etcetera, although I'm certain there are other options for arriving at mutually liked behavior.